Car manufacturers in the United States are currently trying to reduce the overall weight of vehicles in an effort to increase the overall efficiency of fuel consumption at the same time safety bumpers, emission controls, roll bars and other safety features tend to nullify the other weight savings.
The emission controls have greatly reduced the production of vacuum at the intake manifold to an extent that the creation of an operational pressure differential with air at atmospheric pressure in a brake actuation servomotor can be insufficient to stop the vehicle.
In U.S. Pat. Re. No. 28,191, incorporated herein by reference, it is disclosed how air above atmospheric pressure and vacuum can be presented to a servomotor for the creation of an operational braking force. However, under test conditions it has been determined that air at atmospheric pressure and vacuum are present in a sufficient intensity to stop most vehicles during normal braking.
In U.S. Pat. No. 3,780,620, incorporated by reference, it is disclosed how a servomotor may be sequentially presented with air at atmospheric pressure and air above atmospheric pressure by a selected position actuated valve operated by engagement with a plunger. In this servomotor, the air above atmospheric pressure is directly communicated to the rear chamber. In the event that the valve is repeatedly oscillated by an operator within a short period of time, it is possible to deplete the source of vacuum and thereby eliminate the effectiveness of the sevomotor in a braking situation.
In U.S. application Ser. No. 428,495, now U.S. Pat. No. 3,880,049 incorporated herein by reference, it is disclosed how a control plunger may sequentially be positioned to operate a first valve and a second valve in response to an input force. With such a device, it is possible upon an initial application of an input force to bypass the first valve resulting in air above atmospheric pressure rapidly creating the operational pressure differential. However, during such a rapid pressure differential it is difficult to maintain a uniform braking force which provides a comfortable deceleration for most vehicle occupants.